For many practical applications of diode lasers it is desirable to operate the laser in a continuous wave (CW) mode, i.e., at room temperature with a d.c. bias voltage of between about 3 to 6 volts. To achieve CW operation the current density in the active region of the diode laser must reach about 2000 amps/cm.sup.3. Such high current density is difficult to achieve when the active (pumped) area of the laser is large resulting in high total current flow and subsequent overheating.
One of the aforementioned practical applications of diode lasers is utilization as a light source in an integrated optical system using fiber optic elements which may be on the order of only 10 microns in diameter. When the active gain region of the laser is large, several filamentary areas lase over the entire active region. Since the active region is larger than the diameter of the fiber optic elements, the fiber optic elements will transmit light from less than all of the filamentary areas. Thus, power is expended to pump filamentary areas which do not contribute to light output. Also, pumping of used and unused filamentary areas necessitates high pumping current which causes heat dissipation problems.
Several attempts have previously been made to decrease the active region of diode lasers by providing pumping current confinement, as taught by U.S. Pat. Nos. 3,849,790 and 3,920,491. These attempts have been focused on modification of the diode laser structure after completion of the growth of that structure. Specifically, it has been attempted to provide a low resistance current channel through the non-substrate side of a diode laser, with the channel extending to within close proximity to the active region and with a high resistance path on both sides of the channel. The low resistance channel, which can be delineated by ion implantation, diffusion and etching techniques, is formed after growth of the diode laser.
Several problems exist with providing current confinement from the non-substrate side of the diode laser after growth of the laser. First, the low resistance channel must be formed through at least two semiconductor regions which can have variable thickness due to process imperfections, thereby making it difficult to bring the channel into close, uniform proximity to the active region without extending into the active region. Also, the width of the channel is hard to control. Thus, threshold current reduction by operating on the non-substrate side of the diode laser after growth of the diode laser fails to provide reliability in device operation. Also, to provide the low resistance channel an additional continuous layer must be formed on the non-substrate side of the diode. This additional layer moves the metallized contact on the non-substrate side of the diode further from the active region which makes the channel long with the possibility of increased resistance. Also, with the metallized contact further removed from the active region, heat dissipation can be a problem.
More recently, U.S. Pat. No. 3,984,262 proposed providing current confinement by means of a low resistance channel on the substrate side of the diode laser. The channel is formed in the device substrate before the growth of the layers that define the active region of the laser thereby avoiding damage to these layers with the resulting increasing in device reliability. Since the channel is formed on the substrate, its width can be closely controlled and a plurality of channels can be formed simultaneously (with many devices formed from one substrate by subsequent dicing). Current confinement can be further reduced, with the attendant advances of lower pumping current, greater heat dissipation, and single filament operation, by forming current confining channels on both sides of the active region.
Preferably, the current confining channel on the n-side of the diode laser is provided by a diffusion process through a polished surface of a substrate which has masking stripes of nitride material thereupon. The diffused areas form p-n junctions with the substrate material with the channel bounded on both sides by these p-n junctions. Following the diffusion, the remaining layers of the laser diode are grown. Current confinement is achieved because the channel in the substrate allows current to flow while the diffused areas do not conduct due to a reverse bias on the p-n junctions associated with the diffused areas.
As noted, the process of U.S. Pat. No. 3,984,262 requires a diffusion mask, for example, of silicon nitride. The requirement of a diffusion mask complicates the process and may lead to defects due to pin holes in the diffusion mask.